1. Field of the Invention
This present invention relates to a picture printing apparatus, and more particularly to a picture printing apparatus wherein the picture is printed based upon the features of an original picture.
2. Description of the Prior Art
A thermal transfer printing system is a transfer system which transfers the ink of an ink film coated with thermal melting or thermal sublimating ink to the printing paper by a thermal head. The thermal head is heated by applying power to a thermal resistance element. A thermal transfer printing system using the thermal melting ink, is suitable particularly for binary images having only "1" and "0" concentration levels. For multivalved pictures, that is, gradational pictures including halftone pictures, a special concentration modulation technique is necessary. For this concentration modulation technique, a pseudo-halftone printing system for expressing the halftone picture has been attempted by modifying dot density or the ink occupation ratio over a constant area. This is a system widely used to display the gradations with a dot printer that has only a binary concentration expression ability. Typical examples of this method include the dither method, fixed concentration pattern method and multivalved concentration pattern method.
With the dither method, the concentration level of dots in portions of the original picture are compared with threshold levels corresponding to the location of those portions. The dots of a given portion are only printed when the threshold level is exceeded. The dither method is a method to make multivalved pictures in a binary way using threshold patterns (shown in FIG. 1), and the thermal energy supplied to every dot is the same. Therefore this dither method prints a picture in accordance with a variable gradation by controlling the number of printing dots and the ink occupation ratio in a given part of the picture.
With the fixed concentration pattern method, dots are printed according to the element pattern shown in FIG. 2 based on the dot concentration of the corresponding picture element of the original picture.
A multivalved concentration pattern method has been developed by the present inventors, as described in U.S. patent application Ser. No. 821,954, now U.S. Pat. No. 4,724,446. With the dither method and the fixed concentration pattern method, it is difficult to obtain clear and natural pictures taking into account the visual characteristics of a human being. Further, owing to the adverse influence of the thermal storage effect in the thermal head, discontinuities of gradation are formed in the medium and high concentration areas, uneveness results from unstable ink adhesion in the low and medium concentration areas, and so on.
According to this multivalved concentration pattern method developed by the present inventors, the thermal storage effect of the thermal head that was considered conventionally to be a hindrance in producing good prints, can be positively utilized, and smooth pictures of high quality can be produced with a stabilized gradation.
This multivalved concentration pattern method utilizes three dot patterns "A," "B," and "C", corresponding to the low, medium and high concentration levels, respectively, and also corresponding to the energy areas supplied to the thermal head. The energy areas are mutually related to the printing concentration.
For example, as shown in FIGS. 3 and 4, the low concentration fixed pattern "A" is an isolated dot pattern formed by isolating one dot in a picture element, and the gradations in the low concentration area are expressed by modulating the printing energy (energy supplied to the thermal head) and changing the dot size. The medium concentration fixed pattern "B" is a stripe shaped pattern extending in the relative transfer direction (subscanning direction) between the thermal head and printing paper, and the gradations in the medium concentration area are expressed by changing the stripe width by the energy supplied to the thermal head. Furthermore, the high concentration fixed pattern "C" is an L-shaped pattern combined with a white section of dimension 2.times.2, and the gradations of high concentration are expressed by changing the size of the white section in proportion to the remaining portion of the L-shaped dot pattern.
The concept of this multivalved concentration method was disclosed by the present inventors in Proceedings of 3rd Japanese Symposium on Non-Impact Printing Technologies reporting the results of the Symposium held on Jul. 24 and 25, 1986.
Either the multivalved concentration pattern method or the fixed concentration pattern method may be used for good quality printing of multivalved pictures, but if the multivalence pattern method is used for printing binary pictures, poor quality pictures result because the matrix is enlarged. For this reason, mixed multivalved and binary pictures cannot be printed clearly, because the binary portion has such a low resolution.
Thus, there is a problem of decreased resolution when binary pictures are printed by using the fixed concentration pattern method or the multivalved concentration pattern method.
Another printing method was proposed in Laid Open Japanese Application No. 1986-82577 for printing mixed pictures of the multivalved and binary type. In this system, pictures are printed by either the binary printing method or the dither method, or a combination of the two, with halftone pictures being printed by the dither method. All gradational parts are printed by the dither method. Thus, due to the influence of the thermal storage effect in the medium and high concentration areas, all of the adverse effects described previously are encountered.